1. Field of the Invention
The present invention relates generally to a method for fabricating circuit board structure, and more particularly to a circuit board structure with a conductive pillar formed therein by electroplating and electrically connected to circuit layers on surfaces of the circuit board, and method for fabricating the circuit board structure.
2. Description of Related Art
Along with development of the semiconductor package technology, to meet operational demands of microprocessors, chipsets and graphic chips, functions of circuit boards in transmitting chip signal, improving frequency band and controlling impedance must be improved so as to meet development of semiconductor packages with large I/O counts. On the other hand, with R&D effort gradually focused on semiconductor packages of small dimension, multi-function, high speed and high frequency, circuit boards used in semiconductor chip packages are required to have fine pitch and small apertures. The circuit dimension such as line width, circuit space and aspect ratio in current circuit board fabricating process is being reduced.
To increase circuit density of circuit boards used in semiconductor chip packages, a build-up technique is proposed, through which multiple dielectric layers and circuit layers are alternately stacked on a core circuit board and a plated through hole (PTH) is formed penetrating the core circuit board so as to electrically connect the circuits on upper and lower surfaces of the core circuit board. Therein, the fabricating process of the core circuit board is a key factor in influencing the circuit density of the circuit boards.
FIGS. 1A to 1D show a method for fabricating a core circuit board. First, as shown in FIG. 1A, a dielectric layer 100 is provided, which is arranged sandwich-like between thin metal layers 101 made of such as copper foil. A plurality of through holes 102 is formed in the dielectric layer 100. Then, as shown in FIG. 1B, circuit layers 103 are formed on surfaces of the dielectric layer 100 by copper plating and patterning and meanwhile a metal layer is deposited on the walls of the through holes 102. Subsequently, as shown in FIG. 1C, the through holes 102 are filled with a conductive or non-conductive filling material 11 such as dielectric ink or copper-containing conductive paste so as to form plated through holes 102a for electrically connecting the circuit layers 103 on the upper and lower surfaces of the dielectric layer 100. Finally, as shown in FIG. 1D, the superfluous filling material 11 is removed through a buffing process so as to keep flatness of the circuit surface. Thus, a core circuit board 10 is obtained.
FIG. 2 is an upper view of a plated through hole 102a and a circuit layer 103 connected thereto. As shown in FIG. 2, the plated through hole 102a is electrically connected to the circuit layer 103 through an annular metal ring 102b and the inside of the plated through hole 102a is filled with the filling material 11. Taking into account of alignment accuracy, diameter of the annular metal ring 102b is required to be greater than that of the plated through hole 102a, for example, if the diameter of the plated through hole 102a is 200 μm, the diameter of the annular metal ring 102b is required to be 350 μm, which however limits the space for layout of the circuit layer 103 on the core circuit board 10. Moreover, due to great dimension of the annular metal ring 102b, spacing between adjacent plated through holes on the core circuit board is difficult to be reduced, thus preventing high density circuit layout on the core circuit board.
In addition, as the plated through hole 102a is filled with the filling material 11, the final circuit board structure comprises several materials of different coefficients of thermal expansion (CTEs). As a result, delamination can occur in the circuit board structure, thereby adversely affecting reliability of the circuit board structure. Further, as the filling material 11 is easy to absorb moisture, the circuit board quality is also decreased. The through hole filling process and the buffing process also complicate the fabricating process of circuit boards.
Therefore, there is a need to provide a method for fabricating circuit board structure that can overcome the conventional drawbacks such as low density circuit layout, low reliability and quality of circuit boards and complicated fabricating process.